U.S. Pat. No. 5,565,473 discloses the compound of the formula (a): ##STR2## now generally known as montelukast sodium. Montelukast sodium is a leukotriene antagonist and is currently undergoing clinical trials for the treatment of chronic asthma.
PCT Published Application WO96/40638 published Dec. 19, 1996 discloses compounds of formulae (b) and (c), and their individual optical isomers, which are metabolites of montelukast sodium and are themselves leukotriene antagonists. ##STR3##